Add a function to detect patches that can't be safely applied.

pending_manager.py

 # coding=utf8
 # Copyright (c) 2010 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Commit queue manager class.
 
 Security implications:
 
 The following hypothesis are made:
 - Commit queue:
@@ skipping 247 matching lines @@
     if message:
       self.rietveld.add_comment(pending.issue, message)
     try:
       self.queue.pending_commits.remove(pending)
     except ValueError:
       pass
 
   def _apply_patch(self, pending, revision):
     """Applies the pending patch to the checkout and throws if it fails."""
     patch_data = urllib2.urlopen(pending.patch_url()).read()
+    error = patch.verify_patch(patch_data)
+    if error:
+      raise base.DiscardPending(pending, 'Can\'t process patch: %s' % error)
     patch_data = patch.auto_mangle_git_patch(patch_data)
     if not self.checkout.apply_patch(patch_data):
       raise base.DiscardPending(pending,
           'Patch failed to apply against %s.' % revision)
 
   def load(self, filename):
     """Loads the commit queue state from a JSON file."""
     self.queue = model.load_from_json_file(filename)
     self.queue.pending_commits = self.queue.pending_commits or []
 
   def save(self, filename):
     """Save the commit queue state in a simple JSON file."""
     model.save_to_json_file(filename, self.queue)